1. Field of the Invention
The invention is related to the field of communications and, in particular, to inserting timestamps for user equipment in charging messages to provide improved offline charging in IMS networks.
2. Statement of the Problem
One type of communication network gaining popularity is an IP Multimedia Subsystem (IMS) network. As set forth in the 3rd Generation Partnership Project (3GPP) or 3GPP2, IMS provides a common core network having access-agnostic network architecture for converged networks. Service providers are accepting this architecture in next generation network evolution. The IMS architecture is initially defined by the 3GPP to provide multimedia services to mobile subscribers over an Internet Protocol (IP) network, as IP networks have become the most cost savings bearer network to transmit video, voice, and data. The signaling used within IMS networks is typically Session Initiation Protocol (SIP). IMS defines the standard SIP interface between application servers, the IMS core network (CSCF), the IMS subscriber, the IMS database (HSS), and IMS billing elements. These standards can reduce the network integration costs and let the subscriber enjoy more stable services.
For a typical call (or session) within an IMS network, user equipment of a calling party initiates the call through an access network, such as a CDMA network, a GSM network, an IP network, a WiFi network, a WiMAX network, etc. The access network then routes the call to the IMS network. If the access network does not use the same protocols as the IMS network (e.g., SIP and IP), then the access network may route the call to the IMS network through an appropriate gateway. A serving-call session control function (S-CSCF) in the IMS network receives and serves the call in the appropriate manner. The S-CSCF may also contact one or more application servers (AS) in the IMS network to provide services for the call, such as voicemail, call forwarding, etc. To provide charging for the call, each of the IMS nodes (e.g., S-CSCF and AS) handling the call generates charging messages typically in Diameter protocol (Rf). In addition to other call related data, the IMS nodes insert a timestamp into the charging messages. The timestamp is based on a local time for the IMS node.
One problem with present offline charging in IMS networks is that a service provider cannot provide variable-rate billing, such as free nights and weekends, according to the present standards. Because the timestamp inserted in the charging messages is based on a local time for the IMS node generating the charging message, the billing system does not know when a time or day threshold is reached for a particular subscriber. For instance, if a subscriber is located in the Pacific Time zone and an IMS node generating the charging message is located in the Eastern Time zone, then the billing system cannot presently determine when the subscriber has reached a time threshold to allow for free nights and weekends. The billing system only has the local time for the IMS node which may not correlate to the local time for the subscriber. Service providers are thus forced to use flat-rate billing instead of allowing for variable-rate billing.